Sync gigasecond docs with problem-specifications

[kytrinyx]

The gigasecond exercise has been overhauled as part of a project to make practice exercises more consistent and friendly.

For more context, please see the discussion in the forum, as well as the pull request that updated the exercise in the problem-specifications repository:

• https://forum.exercism.org/t/new-project-making-practice-exercises-more-consistent-and-human-across-exercism/3943
• https://github.com/exercism/problem-specifications/pull/2206